Bradycardia is a condition of the heart where the heart beat slows to a rate that is considered insufficient to pump an adequate supply of blood through a patient's body. A heart rate of less than 50 beats per minute is considered as a bradycardia condition for most patients.
One common technique for treating bradycardia is to implant a pacemaker in the patient. The pacemaker senses cardiac electrical activity, which electrical activity normally accompanies a heart beat. If the cardiac electrical activity is not sensed, it indicates that the heart is not beating at a prescribed rate. Stimulation pulses are then generated and delivered to an appropriate heart chamber, either the atrium or the ventricle, in order to stimulate the muscle tissue of the heart to contract, thereby forcing the heart to beat at a rate that is faster than the intrinsic rate. A pacemaker operating to maintain the heart rate at a rate that is faster than a bradycardia rate is referred to as a bradycardia-support pacemaker.
Bradycardia-support pacing is realized in a pacemaker by defining a period of time, referred to generally as the "escape interval," that is slightly longer than the period of time between heart beats of a heart experiencing bradycardia. For example, if the heart is beating at a rate of 50 beats per minute, the time period between consecutive heart beats is 1200 milliseconds. Thus, in a bradycardia-support pacemaker, if it is desired that the heart rate never slow to a rate less than 50 beats per minute, the escape interval of the pacemaker is set to an appropriate value that causes a stimulation pulse to always be generated if more than 1200 milliseconds elapse since the last sensed heart beat. If a heart beat occurs before 1200 milliseconds have elapsed, then that indicates the heart is beating at a rate faster than 50 beats per minute, and no stimulation pulse need be generated. Upon electrically sensing such a "natural" (nonstimulated) heart beat within the allotted time period, the escape interval is reset, and a new escape interval is started. A stimulation pulse will be generated at the conclusion of this new escape interval unless a natural heart beat is again sensed during the escape interval. In this way, stimulation pulses are generated "on demand," i.e., only when needed, in order to maintain the heart rate at a rate that never drops below the rate set by the escape interval.
The heart rate is monitored by examining the electrical signals corresponding to the depolarization of the cardiac muscle tissue. The depolarization of the cardiac muscle tissue triggers the mechanical contraction of the cardiac muscle tissue. The electrical signal corresponding to the depolarization of the atrial muscle tissue is identified as a P-wave on a surface EKG. The electrical signal corresponding to the depolarization of the ventricular muscle tissue is identified as an R-wave on a surface EKG. The sequence of electrical signals, corresponding to P-waves followed by R-waves, can be sensed from inside of or directly on the heart by using sensing leads having appropriate electrodes that are implanted inside or on the heart, e.g., pacemaker leads. The electrical signals corresponding to P-waves and R-waves sensed internal to or directly on the heart are referred to as the electrogram (EGM) of the heart.
A pacemaker includes means for sensing P-waves and/or R-waves, and hence means for monitoring the patient's EGM. From such EGM, the physical activity of the heart (i.e., a muscle contraction of a given heart chamber, atrium and/or ventricle) can be deduced. In order to determine the heart rate, for example, the pacemaker measures the time that elapses between consecutive R-waves. The R-wave is normally used for this determination because the R-wave is normally a much larger electrical signal than the P-wave, and is hence much easier to sense. However, the same rate determination can also be made by measuring the time between consecutive P-waves, if desired.
R-waves and/or P-waves are sensed by placing an electrode in contact with, or in proximity to, the cardiac tissue of interest. Most pacemakers use the same electrode for sensing R-waves and/or P-waves, as is used to deliver stimulation pulses to the ventricle and/or atrium, respectively, although separate sensing and stimulation electrodes could and have been used. In order to prevent electrical noise or other low level electrical signals from being sensed as electrical cardiac activity when in fact cardiac electrical activity (e.g., an R-wave) has not occurred, it is necessary to define a threshold level above which the amplitude (and/or other characteristics) of a sensed electrical signal must go before such signal is recognized as an indicator of cardiac electrical activity. Unfortunately, the use of threshold detection in this manner sometimes precludes the detection of a valid low-level R-wave, or other valid cardiac electrical signal, that is below the set threshold level. While every attempt is made to set the threshold level so as to minimize missing the detection of valid cardiac electrical signals, the threshold level cannot be set so low so as to commonly detect noise or other invalid signals as valid signals. Hence, a tradeoff must be made, and usually such tradeoff favors not sensing noise or other invalid signals, thus potentially missing valid low level cardiac electrical signals. Hence, as a practical matter, most pacemakers are set such that they will occasionally fail to sense a valid low level R-wave and/or P-wave.
It is noted that all modern implantable pacemakers are programmable. That is, the basic escape interval of the pacemaker, as well as the threshold level of the sensing circuits used in the pacemaker, as well as numerous other operating parameters of the pacemaker, may be programmably set at the time of implantation or thereafter to best suit the needs of a particular patient.
Recently, there has also been much interest shown in implantable cardioverter defibrillation (ICD) systems. An ICD system provides one or more high energy shocking pulses to a heart when: (1) the ICD senses that the heart is beating fast (tachycardia); or (2) the ICD senses that the heart is beating in a rapid, chaotic manner (fibrillation). (Note, that an ICD device senses electrical cardiac activity, just as does a pacemaker, and determines the heart rate by measuring the time interval between consecutive R-waves. When the ICD senses ventricular fibrillation--a very rapid, chaotic R-wave rate--the mechanical effect on the heart is cardiac arrest, i.e., the heart muscles do not contract effectively, and blood is not pumped through the body.)
It is noted that the high energy shocking pulse delivered by an ICD device has an energy content on the order of joules, whereas the stimulation pulse delivered by a pacemaker has an energy content on the order of microjoules. In order to clearly distinguish the low energy pacing pulses of a pacemaker from the high energy shocking pulses delivered by an ICD device, the pacemaker pulses will be referred to herein as "stimulation pulses" and the ICD pulses will be referred to as "shocking pulses."
The purpose of delivering a high energy shocking pulse during ventricular tachycardia, ventricular fibrillation, or other tachyarrhythmias is to break or stop the tachycardia, fibrillation, or other tachyarrhythmia. Tachycardia, fibrillation, and other tachyarrhythmias are sustained by an imbalance in the recovery and conduction among the various tissue of a given heart chamber, typically the ventricle. Such imbalance is referred to as temporal dispersion of refractoriness. The high energy shocking pulse depolarizes any tissue which is not depolarized at that moment. That is, it puts all of the tissue (or a large percentage of the tissue) into the same physiologic state (depolarized) and thus, when such tissue recover or repolarize, it will be able to be activated or depolarized in a synchronized or coordinated manner.
In the case of a tachycardia, the delivery of the shocking pulse or pulses by the ICD is usually referred to as "cardioversion," and the shocking pulse is typically delivered in synchrony with the heart's R-wave in order to avoid delivering the shocking pulse to the heart during the T-wave portion of the cardiac cycle. (The T-wave portion is that portion of the cardiac cycle, following the R-wave during which the massive ventricular tissue is repolarizing.) The reason that one tries to avoid delivering a high energy shocking pulse (or even a low energy shocking pulse or a pacemaker stimulation pulse) onto the T-wave is that such action could have a paradoxical effect and further accelerate the heart rhythm. That is, a slow ventricular tachycardia might be accelerated to a faster ventricular tachycardia, and/or a faster ventricular tachycardia might be accelerated to ventricular fibrillation.
In the case of fibrillation, there is a chaotic and rapid beating of the many individual muscle fibers of the heart, and the heart is consequently unable to maintain effective synchronous contraction, and is thus not able to pump blood. For all practical purposes, the heart has mechanically stopped, although (as indicated above) electrically it is very active with multiple chaotic electrical signals. Hence, the purpose of delivering a shocking pulse or pulses to the heart during fibrillation (also commonly referred to as "defibrillation" pulses) is synchronize or coordinate the cardiac tissue so that the many individual muscle fibers can once again maintain effective synchronous contractions, and thereby efficiently pump blood through the patient's body.
Conventional ICD devices known in the art typically include a built-in sensor circuit. Such sensor circuit is designed to sense, through attached sensing electrodes, the rate at which the heart is beating. If the sensed heart rate exceeds a high fixed rate threshold (i.e., if a tachycardia is sensed), the ICD is designed to deliver a low energy shocking pulse, commonly referred to as a cardioversion pulse. If fibrillation is detected, the ICD is designed to deliver a high energy shocking pulse, or shocking pulse. Typically, a cardioversion pulse will be a lower energy discharge than will a shocking pulse.
For a patient having both a bradycardia support pacemaker and an ICD device, a potential problem occurs when there is a low amplitude fibrillation that is not recognized by the sensing circuits of the pacemaker (i.e., a very fast chaotic heart rhythm that is of such a low amplitude that the sensing circuits of the pacemaker cannot sense it). As far as the pacemaker is concerned, no cardiac activity is occurring because none is sensed. If the ICD is able to "see" these signals, it will respond appropriately. If, however, the ICD also fails to "see" the low amplitude fibrillation signals (and does not respond), then the pacemaker circuits will interpret this lack of activity as asystole and release an output stimulation pulse. If the patient is really fibrillating, such output stimulation pulse will be ineffective. However, if the ICD device now "sees" (i.e., senses) the output stimulation pulse, its circuits will interpret such ineffective stimulation pulse as an R-wave, and will thus not charge up nor release a shocking pulse. Furthermore, if the ICD does not "see" the low amplitude ventricular fibrillation, it will remain quiescent. Hence, the fibrillation goes undetected and untreated.
For a patient having a combination implantable stimulation device that includes both the bradycardia, cardioversion and defibrillation functions, a similar problem exists in that the electrogram signal from the ventricular fibrillation may be so low in amplitude that neither the ICD nor the pacemaker sensing circuits sense anything, thus causing the pacemaker portion of the system to release a stimulation pulse. Upon releasing the stimulus, the automatic gain feature of the ICD/pacemaker sensing circuits, if enabled, incrementally increases its sensitivity to its most sensitive setting, in an attempt to "look" for an R-wave. If a failure to sense an R-wave persists, the diagnosis is "true asystole," and the ICD/pacemaker will continue to release stimulation pulses at its programmed rate. Unfortunately, if the rhythm is truly ventricular fibrillation with a EGM signal that is too low to be sensed by either the pacemaker portion of the device or the ICD component, the stimulation pulses of the pacemaker will not be effective. However, the pacemaker does not know that its pacing stimulation pulses are ineffective, so it will just continue to deliver such ineffective pacing stimuli.
What is needed, therefore, is a pacemaker/ICD device or system wherein a proper response to an alleged asystole can occur, and wherein the pacemaker/ICD device can ascertain whether or not a given stimulation pulse is effective, i.e., whether it "captures" the heart.